In 3GPP-LTE (3rd Generation Partnership Project Radio Access Long Term Evolution, hereinafter referred to as “LTE”), OFDMA (Orthogonal Frequency Division Multiple Access) is used as a downlink communication method, and SC-FDMA (Single Carrier Frequency Division Multiple Access) is used as an uplink communication method (see Non-Patent Literatures 1, 2, and 3, for example).
In LTE, a radio communication base station apparatus (hereinafter abbreviated to “base station”) performs communication by assigning resource blocks (RB) within a system band to a radio communication terminal apparatus (hereinafter abbreviated to “terminal”) in time units called “subframes.” Also, a base station transmits assignment control information (L1/L2 control information) for notifying downlink data and uplink data resource assignment results to a terminal. This assignment control information is transmitted to a terminal using a downlink control channel such as a PDCCH (Physical Downlink Control Channel), for example. Here, each PDCCH occupies a resource comprising one or a continuous plurality of CCEs (Control Channel Elements). In LTE, a number of CCEs occupied by a PDCCH (linked number of CCEs: CCE aggregation level) is selected as one of 1, 2, 4, or 8, according to the number of information bits of assignment control information or the channel state of a terminal. In LTE, a frequency band having a maximum width of 20 MHz is supported as a system bandwidth.
Also, a base station transmits a plurality of PDCCHs simultaneously in order to assign a plurality of terminals to one subframe. At this time, the base station transmits a CRC bit masked (or scrambled) by a transmission-destination terminal ID, included in a PDCCH, in order to identify a transmission-destination terminal of each PDCCH. Then a terminal performs blind decoding of a PDCCH by demasking (or descrambling) a CRC bit with that terminal's terminal ID in a plurality of PDCCHs for which there is a possibility of that terminal being addressed.
Furthermore, assignment control information transmitted from the base station is called “DCI (Downlink Control Information)” and includes information on resources assigned by the base station to the terminal (resource assignment information) and MCS (Modulation and channel Coding Scheme) or the like. The DCI has a plurality of formats for uplink, for downlink MIMO (Multiple Input Multiple Output) transmission and for downlink non-continuous band assignment or the like. The terminal needs to receive both downlink assignment control information (downlink-related assignment control information) and uplink assignment control information (uplink-related assignment control information) having a plurality of formats.
For example, the downlink assignment control information defines formats in a plurality of sizes according to a transmitting antenna control method and resource assignment method or the like of the base station. Of the plurality of formats, a downlink assignment control information format for performing continuous band assignment (hereinafter simply referred to as “downlink assignment control information”) and an uplink assignment control information format for performing continuous band assignment (hereinafter simply referred to as “uplink assignment control information”) have the same size. These formats (DCI formats) include type information (e.g., 1-bit flag) indicating the type of assignment control information (downlink assignment control information or uplink assignment control information). Thus, even when the DCI size indicating the downlink assignment control information and the DCI size indicating the uplink assignment control information are the same, the terminal can identify whether the assignment control information is the downlink assignment control information or uplink assignment control information by checking the type information included in the assignment control information.
The DCI format used when uplink assignment control information for performing continuous band assignment is transmitted is called “DCI format 0” (hereinafter referred to as “DCI 0”) and the DCI format used when downlink assignment control information for performing continuous band assignment is transmitted is called “DCI format 1A” (hereinafter referred to as “DCI 1A”). As described above, DCI 0 and DCI 1A have the same size and can be distinguished by type information, and therefore DCI 0 and DCI 1A will be represented collectively as “DCI 0/1A.”
In addition to the above-described DCI formats, there are DCI format 1 (hereinafter referred to as “DCI 1”) for performing non-continuous band assignment on a downlink and DCI formats 2 and 2A (hereinafter referred to as “DCI 2 and 2A” for assigning spatial multiplexing MIMO transmission. Here, DCI 1, 2 and 2A are formats used in dependence on the downlink transmission mode of the terminal (non-continuous band assignment or spatial multiplexing MIMO transmission) and are formats set for each terminal. On the other hand, DCI 0/1A is a format independent of the transmission mode, format that can be used for a terminal in any transmission mode, that is, format that can be used commonly for all terminals. Furthermore, when DCI 0/1A is used, 1 antenna transmission or transmission diversity is used as a default transmission mode.
Furthermore, a method has been investigated that limits CCEs subject to blind decoding for each terminal in order to decrease the number of blind decoding operations to reduce the circuit scale of a terminal. With this method, a CCE area (hereinafter referred to as “search space”) that is subject to blind decoding is limited for each terminal. In LTE, a search space is set randomly for each terminal, and a number of CCEs included within a search space is defined for each PDCCH CCE aggregation level. For example, for CCE aggregation levels 1, 2, 4, and 8, respectively, the number of CCEs included within a search space, that is, the number of CCEs subject to blind decoding, is limited to six candidates (6 (=1×6) CCEs), six candidates (12 (=2×6) CCEs), two candidates (8 (=4×2) CCEs), and two candidates (16 (=8×2) CCEs), respectively. By this means, each terminal need only perform blind decoding on CCEs within a search space assigned to that terminal, enabling the number of blind decoding operations to be decreased. Here, a search space of each terminal is set using a terminal ID of each terminal, and a hash function, which is a function that performs randomization. This terminal-specific CCE area is called “UE specific Search Space (UE-SS).”
On the other hand, a PDCCH also includes control information for data assignment common to terminals simultaneously reported to a plurality of terminals (e.g., assignment information related to a downlink broadcast signal and assignment information related to a paging signal) (hereinafter also referred to as “control information for shared channels”). In order to transmit control information for shared channels, a CCE area (hereinafter also referred to as “Common Search Space: C-SS”) common to all terminals that should receive a downlink broadcast signal is used for a PDCCH. In C-SS, for CCE aggregation levels 4 and 8, respectively, there are four candidates (16(=4×4) CCEs) and two candidates (16=(8×2) CCEs), a total of six candidates for CCEs subject to blind decoding.
Furthermore, the terminal performs blind decoding on each of DCI formats in two sizes in a UE-SS; DCI format (DCI 0/1A) commonly used for all terminals and DCI formats (DCI 1, 2, 2A) dependent on a transmission mode. For example, the terminal performs 16 blind decoding operations for each of PDCCHs in two sizes within a UE-SS. Furthermore, the terminal performs six blind decoding operations described above on each of DCI format 1C (hereinafter also referred to as “DCI 1C”) which is a format for shared channel assignment and DCI 1A (that is, a total of 12 blind decoding operations).
Here, DCI 1A used for shared channel assignment and DCI 0/1A used for terminal-specific data assignment have the same size and are distinguished from each other by terminal IDs. Therefore, the base station can transmit DCI 0/1A for performing terminal-specific data assignment also with a C-SS without increasing the number of blind decoding operations by the terminal.
Also, standardization has begun on 3GPP LTE-Advanced (hereinafter referred to as “LTE-A”), which implements still higher communication speeds than LTE. In LTE-A, a maximum downlink transmission speed of 1 Gbps or above and a maximum uplink transmission speed of 500 Mbps or above are implemented, offering the prospect of base stations and terminals (hereinafter referred to as “LTE-A terminals”) capable of communication at a wideband frequency of 40 MHz or above being introduced. Also, an LTE-A system is required to accommodate not only LTE-A terminals but also terminals compatible with an LTE system (hereinafter referred to as “LTE terminals”).
In LTE-A, a band aggregation method has been proposed whereby a plurality of frequency bands are aggregated in performing communication in order to implement wideband communication of 40 MHz or above (see Non-Patent Literature 1, for example). For example, a frequency band having a width of 20 MHz is assumed as a basic communication band unit (hereinafter referred to as a “component band”). Therefore, in LTE-A, for example, a 40 MHz system bandwidth is implemented by aggregating two component bands. Also, both an LTE terminal and an LTE-A terminal can be accommodated in one component band.
In LTE-A, when data is assigned to a plurality of component bands for a certain terminal, assignment control information is notified through a plurality of PDCCHs. That is, the resource assignment result of a plurality of component bands is notified using one PDCCH for each component band.
In LTE-A, a transmission method using non-continuous band assignment and a transmission method using MIMO are newly introduced as uplink transmission methods. As a result, studies are being carried out on a definition of new DCI formats (e.g., DCI formats 0A, 0B (hereinafter also referred to as “DCI 0A and 0B”)) (see Non-Patent Literature 4, for example). That is, DCI 0A and 0B are DCI formats in dependence on an uplink transmission mode.